Renal System MCAT Practice Question
A 68-year-old male with a history of lithium use for bipolar disorder presents with polyuria and polydipsia for 3 months. Physical examination reveals dry mucous membranes and mild orthostatic hypotension. Laboratory studies show: serum osmolality 305 mOsm/kg, serum sodium 148 mEq/L, and plasma ADH 28 pg/mL (normal 2-8). A water deprivation test is performed. After 8 hours of fluid restriction, serum osmolality increases to 312 mOsm/kg while urine osmolality remains 180 mOsm/kg. Administration of desmopressin (synthetic ADH) does not increase urine osmolality above 190 mOsm/kg. Which of the following is the most likely diagnosis?
Answer choices
- ANephrogenic diabetes insipidusCorrect answer
- BCentral diabetes insipidus
- CPrimary polydipsia
- DSyndrome of inappropriate ADH secretion
- EAcute tubular necrosis
- FMedullary interstitial fibrosis from chronic pyelonephritis
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